CH. Ill] EXPERIMENTS IX CROSS-FERTILIZATIOX 29 



Born obtained some interesting results as to the relations 

 existing between the number of spermatozoa in a fluid-extract 

 of the testis and the power of the fluid-extract to fertilize eggs. 

 He insists that in some cases there is a necessary connection 

 between the two. It is far from clear how this is possible, and 

 the result may depend on other causes which are introduced 

 along with the solutions employed. Moreover, the further 

 question of polyspermy of such eggs complicates the results. 

 Born believes that many cases of irregular segmentation of 

 crossed eggs are due to the entrance of several or many sper- 

 matozoa into the egg, which act as centres for protoplasmic 

 accumulations. Such a segmentation he calls "barock" seg- 

 mentation. On the other hand, Pfliiger suggests that the 

 irregular cleavage of certain of the crossed eggs is the result 

 of the disintegration of the male pronuclei, so that the chro- 

 niatin is scattered, and then acts on the protoplasm, producing 

 an irregular division. 



Recent results have shown that polyspermy is a normal oc- 

 currence in some amphibian eggs, and, despite the presence 

 of several spermatozoa, normal cleavage and normal embryos 

 result. The changes that take place within the cross-fertilized 

 eggs must be more carefully studied before a final decision can 

 be reached in regard to the meaning of some of the experiments 

 described above. 



We must not confuse two factors that enter into the problem 

 of cross-fertilization. On the one hand, the spermatozoon may 

 not be able to push through the gelatinous coatings of the egg, 

 or it may not be able to bore through the outer surface of the 

 egg itself, or it might be unable to enter the protoplasm if the 

 latter were entirely free from its coats. 1 On the other hand, 

 even if the spermatozoon could successfully enter and combine 

 with the female pronucleus, it does not follow that the egg 

 would develop. We now know that so many factors enter 

 into the problem of fertilization of the egg that it is not sur- 

 prising when we find that two pronuclei that have ever so 

 slight differences are not able to carry out the complicated 

 machinery of cell-division and development. 



1 As in the case of naked pieces of protoplasm of the egg of species of sea-urchins. 



